Merge pull request #3596 from sveltejs/digital-climate-strike

add digital climate strike banner

Author: Rich-Harris

site/src/template.html

@@ -16,6 +16,8 @@
 	<meta name='twitter:creator' content='@sveltejs'>
 	<meta name='twitter:image' content='https://svelte.dev/images/twitter-card.png'>
 
+	<script src="https://assets.digitalclimatestrike.net/widget.js" async></script>
+
 	<!-- Sapper generates a <style> tag containing critical CSS
 	     for the current page. CSS for the rest of the app is
 	     lazily loaded when it precaches secondary pages -->